Cytosolic Ca2+ plays a key role in intracellular signaling, and in particular, in regulating proliferation/differentiation in a number of cell types. In many instances, as exemplified by cardiac muscle contraction and the transient release of neurotransmitter substances, the elevated cytosolic free Ca2+ concentration may be required for only brief periods of time. The Ca2+ must then be rapidly removed from the cytosol by a means such as a Na+/Ca2+ exchanger, a Ca2+-selective ion channel, or an ATP-driven Ca2+ pump.
The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane of all animal cells (for reviews, see, e.g., Blaustein & Lederer (1999). Physiol. Rev 79: 763–854). It extrudes Ca2+ in parallel with the plasma membrane ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the plasma membrane, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). See, for example, Nicoll et al. (1990) Science 250: 562–565; Li et al. (1994) J. Biol. Chem. 269: 17434–17439; and Nicoll et al. (1996) J. Biol. Chem. 271: 24914–24921.
At the structural level, all three Na+/Ca2+ exchangers are modeled to have 11 transmembrane segments with a large intracellular loop between the fifth and sixth transmembrane segments. See, e.g., Hilgemann et al. (1991) Nature 352: 715–718. At the functional level, only NCX1 has been well characterized. Mutagenesis experiments indicate that specific regions of the transmembrane segments are critical in the ion transport process (Nicoll et al. (1996) J. Biol. Chem. 271: 13385–13391). The large intracellular loop of NCX1 is primarily involved in various regulatory properties, such as intracellular Na+-dependent inactivation and Ca2+-dependent regulation. See, e.g., Hilgemann (1990) Nature 344: 242–245. In addition, each of the NCX exchangers has distinctive putative phosphorylation sites, although roles for each of these sites have not been elucidated. NCX1 of rat smooth and cardiac muscle is phosphorylated by protein kinase C, with a resultant modest stimulation of transport activity (Iwamoto et al. (1996) J. Biol. Chem. 271: 13609–13615). NCX1, NCX2, and NCX3 have similar consensus sites for tyrosine phosphorylation.